Nitriding of iron and steel parts in salt bath having improved corrosion resistance

ABSTRACT

A new nitriding process by using a salt bath to produce iron and steel parts having excellent abrasion resistance and corrosion resistance includes forming an iron lithium complex oxide layer at the outermost surface of the iron part by immersing the iron and steel parts in a salt bath containing cationic component of Li, Na and K and anionic components of CNO − and CO 3   2− , where hydroxide compound selected from lithium hydroxide, sodium hydroxide and potassium hydroxide are added to the salt bath. Materials being in a hydrated state or in a free water containing state can be used for preparation or replenishing of the salt bath. Moistened air of (1×10 −2  kg·H 2 O)/(1 kg dry air) can be used for mixing the salt bath. Containing ratio of Li, Na, K is preferable where a solidifying temperature of the mixture of carbonates of Li, Na, K in that ratio is lower than 500° C. It is preferable that the mol ratio of Na and K is to be 2:8˜8:2, the content of CNO −  is to be 5˜35 wt %, the content of CN −  in the salt bath is less than 2 wt % and the temperature of the salt bath is to be 450˜650° C.

FIELD OF THE INVENTION

This invention relates to an improvement of corrosion resistance of ironand steel parts obtained by nitriding in a salt bath, which alsoprovides high abrasion resistance and high strength against fatigue.

PRIOR ART

Nitriding process in salt bath, which forms a nitrided layer on asurface of iron and steel materials, has been utilized to improvestrength of the surface of those iron and steel materials,thereby toenhance abrasion resistance and strength against fatigue of thosematerials. The nitrided layer formed by the above-described processinghas also a function to prevent a corrosion loss of the materials.Therefore, if it is the case that corrosion resistance of usual improvedlevel is required, this process may be completed by employing aconventional nitriding process in salt bath.

However, for a use where corrosion resistance at a high level as in hardchromium plating is required, which is a competitive surface hardeningprocess, further processing must be made in addition to the nitridingprocess in salt bath.

Improvement of corrosion resistance of iron and steel parts by nitridingprocess has been reported in JP56-33473A, JP60-211062A, JP5-263214A,JP05-195194A, JP7-62522A, JP-7-224388A, etc.

In JP56-33473A and JP07-22438A, a combined processing of nitridingprocess and oxidation bath process is proposed as a method to improvecorrosion resistance. The corrosion resistance obtained by this combinedprocessing was found to be as equivalent or superior than that obtainedby hard chromium plating process in salt water spray test.

However, since the corrosion resistance level obtained by said combinedprocessing with oxidation bath widely varies, this method was usuallynot applied in view of a quality control (lower limit value control ofproducts).

Further, a method of using a wax following a nitriding process andoxidation bath process, and a method to apply a polymer coating, havebeen proposed in, for example, JP05-195194A and JP05-263214A.

The two methods mentioned above are aiming at, as one aspect, loweringan abrasion coefficient of the material and then enhancing abrasionresistance of the material by way of applying either wax or a polymercoating to the material, and, as another aspect, sealing or covering anoxide layer of the material by coating with wax or polymer thereby toenhance corrosion resistance and stability of the material. These twomethods improve and stabilize the material properties, such as abrasionresistance, strength against fatigue and corrosion resistance.

However, it is not an easy way to accept in views of investment,production efficiency and cost to incorporate a process of coating ofwax or polymer in addition to said oxidation bath process following thenitriding. Based on such a background, the following was proposed.

In JP07-62522A, another nitriding method for providing corrosionresistance to iron and steel parts has been proposed. This method formsan oxide layer on the nitrided layer by performing anodic electrolysisduring nitriding process. Since this method requires a single salt bath,it is expected that great advantages in the productivity and productioncost can be attained by replacing of conventional two-step process ofnitriding process and oxidation bath.

However, the process of anodic electrolysis is executed by using theopposite electrode as a cathode. And, due to a cathodic reaction at theopposite electrode, cyanate compound in the salt bath is reduced toproduce cyanide compounds, and accordingly, concentration of thepoisonous cyanide compound in the salt bath tends to be increased thanin the salt bath where no electrolysis is executed.

In addition, for carrying out an appropriate operation, current densityat each site of the iron and steel parts must be controlled in apredetermined range. For this purpose, close attentions are necessary toan arrangement of the electrode and the iron and steel parts to beprocessed. Furthermore, if the iron and steel parts to be processed hasan unsuitable configuration for electrolysis such as having deep holesor bag-shaped holes, employment of this method would be difficult.Therefore, iron and steel parts to be processed by this method must belimited.

Based on the background as described above, there has been a requirementto establish a new method of nitriding process, which comprises singlestep, does not require an electrolysis, and can provide iron and steelparts having satisfactory abrasion resistance and corrosion resistance.

DISCLOSURE OF THE INVENTION

It is disclosed in JP58-77567A that in a nitriding process using a saltbath comprising anionic components of CNO⁻ and CO₃ ²⁻, and two cationiccomponents of Na⁺ and K⁺, an unexpected black-colored film in smut formhaving poor adhesiveness is produced on a surface of the nitrided layerwhen a content of a by-produced cyanide in the salt bath is low. And, itis known that this film in smut form is a magnetite (Fe₃O₄).

The inventors of the present invention carried out more differentnitriding of a steel plate using a salt bath comprising anioniccomponents of CNO⁻ and CO₃ ²⁻ and three cationic components of Li⁺, Na⁺and K⁺, where the content of the by-produced cyanide in the salt bath iskept low. In contrast to the result in JP58-77567A using a salt bathcontaining Na⁺ and K⁺ as the cationic component, inventors have obtaineda black-colored film with satisfactory adhesion to the material.

Then, the processed steel plate by the inventors was subjected to a saltwater spray test to check the corrosion resistance. As a result, thesteel plate by the inventors showed to have high corrosion resistance,namely more than 200 hours are required to cause the rust on the surfaceof the steel plate. With this result, it is judged that theblack-colored film with satisfactory adhesion has a function to protectiron and steel parts.

With regard to the reason why this protective film is formed on thesurface of material in the salt bath containing a low concentration of acyanide product, the inventors are supposing as follows.

1. Since the content of the by-producted cyanide having a reducingproperty is low, oxidizing property of the salt bath is enhanced,thereby causing oxidation of a surface of iron to produce its oxides inparallel to nitriding reaction by a cyanate.

2. Since the concentration of CN⁻ having a strong power to dissolve theiron is low, and since a capability of the salt bath to dissolve ironoxides produced on a surface of iron is lowered, the oxides can beproduced as in 1. above and an oxide film can be formed on the outermostsurface.

The inventors of this invention analyzed the film on the steel plateproduced by the salt bath of three-component of Li⁺, Na⁺ and K⁺ asdescribed above by means of X-ray diffraction.

As a result, it was found that the film produced by the salt bath ofthree alkali metal component including lithium is an iron-lithiumcomplex oxide.

Iron-lithium complex oxides, Li₂Fe₃₃O₄, Li₂Fe₃O₅, Li₅Fe₅O₈,LiFe₅O.sub.8, LiFeO₂, Li₅FeO₄, Li₂Fe₂.4O_(4.6) and the like have beenknown. From the analytical result by X-ray diffraction of the film,Li₂Fe₃O₄, Li₂Fe₃O₅, Li₅Fe₅O.sub.8 and LiFe₅O.sub.8 have been observed sofar.

Reasons why the film of this iron-lithium complex oxide is adhesive andgood in corrosion resistance.

In case of the salt bath of two cationic component of Na⁺ and K⁺, a filmof (magnetite Fe₃O₄) in smut form with poor adhesion is produced on thesteel plate. On the other hand, when the salt bath of three cationiccomponent of Li⁺, Na⁺ and K⁺ is used, a film of iron-lithium complexoxide having satisfactory adhesion property and good in corrosionresistance is formed. The inventors of the present invention havesupposed the reason as follows.

In case of the salt bath of two component of Na⁺ and K⁺, the filmproduced onto the surface of the steel plate is magnetite(Fe₃O₄). Theboth cationic ions of Na⁺and K⁺ have a large ionic diameter. Therefore,they cannot be a constituent component of the oxide layer. Theconstituents of the magnetite are Fe²⁺, Fe³⁺ and O²⁻.

Since these ions are all multiply charged ions, it is difficult for themto simultaneously satisfy a neutralization of electric charges and asuitable positioning of lattice structure during formation of the film.And the formed film has various defects in microscopic and macroscopicviews.

In contrast thereto, the film produced onto the surface of a steel platewhen using the salt bath of three component of Li⁺, Na⁺ and K⁺ is theiron-lithium complex oxide. Since Li⁺ ion has small ionic diameter, itcan be incorporated into the iron oxide film as a constituent, therebythe iron-lithium complex oxide is produced.

Since Li⁺ is a monovalent cation, it has an important function tosimultaneously satisfy a neutralization of charges and a suitablepositioning of lattice structure during formation of a film. By virtueof this function of Li⁺, it is assumed that the film having less defectscan be formed. Incidentally, it is known that Li⁺ can move in the oxideeven at a room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between cyanate concentration andby-producted cyanide concentration in the salt bath containing Li, Naand K.

FIG. 2 is a graph showing an example of the composition of the filmformed by the process according to the present invention.

FIG. 3 is a diagram explaining a preferable range of composition of thesalt bath.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

Based on the general experience of the inventors that a film havingadhesion property and corrosion resistance property can be formed onlyby applying nitriding salt bath where the content of the by-productedcyanide is low in the salt bath containing of anionic components of CNO⁻and CO₃ ²⁻ and cationic components of Li⁺, Na⁺ and K⁺, a test wascarried out in order to find out preferable range of the film-formingprocess.

Since it was supposed that the aimed iron-lithium complex oxide filmwill be formed when the by-producted cyanide in the salt bath is at alow concentration, in this example 1, the content of CNO⁻, which is aparent substance of producing the by-producted cyanide in the salt bath,was set at two concentration levels, that is 35 wt % as a standardconcentration and 15 wt % as low concentration. The composition of thesalt bath is shown in Table 1 below.

TABLE 1 Component in Salt Bath S2-1 S2-2 Li⁺ (mol %) 31 31 Na⁺ (mol %)26.5 26.5 K⁺ (mol %) 42.5 42.5 CNO⁻ (wt %) 35 15 CO₃ ⁻ Balance Balance

60 Kg of salt mixture having composition of S2-1 in Table 1 was placedin a crucible made of titanium having a diameter of 350 mm and a depthof 500 mm to which a pipe for air bubbling was provided, and the mixturewas then melted. 35wt % of CNO⁻ was provided by coverting a carbonateaccording to a process shown in JP54-7502B.

The molten salt bath is maintained at 580° C. while air was blown fromthe bottom at a blowing rate of 150 L/Hr to ensure the homogeneity ofthe salt bath. The test was then carried out by using round bar ofcarbon steel S15C (20 mmφ×8 mmt), cold rolled steel sheet SPCC (50mm×100 mm×0.8 mmt) and iron powder (surface area: 8 m²/120 g) of 60mesh. Iron powder was used for increasing in experiment the processingarea of iron materials. The carbon steel S15C and the cold rolled steelsheet SPCC were immersed in the salt bath for 90 min. at 580° C.,water-cooled, washed with tap water and dried.

The iron powder in an amount of 120 g for each time was added into themolten salt bath 5 times a day at an interval of 90 min. At the time ofthe fifth addition of the iron powder, the carbon steel S15C and thecold rolled steel sheet SPCC were processed. At that time, sampling wasmade from the molten salt bath for the analysis.

At the end of the operation for one day, solid dregs in the molten saltbath were removed. The processing tests were continuously carried outfor 8 days.

The molten salt bath of a composition as shown in S2-2 in Table 1 wasprepared in the same manner except the amount of CNO⁻ is adjusted to 15wt %. Then, the tests were carried out as same as in the case of themolten salt bath of S2-1. FIG. 1 shows the amount of the by-producedcyanide in the salt bath of S2-1 and S2-2 respectively.

It was determined that the content of the cyanide in both of the saltbaths S2-1 and S2-2 at the starting time was zero respectively. In bothsalt baths of S2-1 and S2-2, it was recognized that the content of thecyanide gradually increased with the progress of the test.

In the salt bath of S2-1, the content of the cyanide was 0.4 wt % on thethird day, and it reached to near 1.7 wt % on the eighth day and thecontent is still increasing.

On the other hand, in the salt bath of S2-2, the content of the cyanidewas 0.26 wt % on the third day, it reached to the peak value of 0.54 wt% on the seventh day and then came to the equilibrium on the eighth day.

The external appearance of the carbon steel S15C and the cold rolledsteel sheet SPCC after the test was checked. As a result, in case ofsalt bath of S2-1, a black-colored surface that seems to be containingiron-lithium complex oxide was recognized for both of S15C and SPCCuntil the third day. However, on the fourth day, it changed to a grayishcolor, which is considered to be a nitrided layer, and the grayish colorin the appearance continued until the eighth day.

In contrast thereto, the test pieces of S15C and SPCC processed by thesalt bath of S2-2 presented a black-colored appearance for all of thetest specimen from the first day until the eighth day.

Table 2 shows the results of the salt water spray tests conducted forthe test pieces processed by salt baths of S2-1 and S2-2 in accordancewith JIS Z2371, respectively.

TABLE 2 Test Results Of Corrosion Resistance (Salt Water Spray Test inaccordance with JIS Z2371: Hours until appearance of rust) Salt Day OfSalt Bath Treatment Bath Material 1st 2nd 3rd 4th 5th 6th 7th 8th S2-1S15C >200 >200 >200 48 24 24 24 24 SPCC >200 >200 >200 72 24 24 24 24S2-2 S15C >200 >200 >200 >200 >200 >200 >200 >200SPCC >200 >200 >200 >200 >200 >200 >200 >200

It was noted that there is a close relation between the corrosionresistance and the external appearance. All of the test pieces havingblack-colored appearance showed satisfactory corrosion resistance.

FIG. 2 shows a result of analysis measured on the depth from the surfacefor the SPCC material treated in the salt bath of eighth day of S2-2 at580° C. for 120 min. by means of glow discharge spectroscopy (GDS). Asshown in FIG. 2, an iron-lithium complex oxide film of 2 to 3 μm thickexists on the outermost layer, and the nitrided layer of about 10 μmthick exists under that film.

In order to investigate an industrial life of the salt bath, theinventors of the present invention proceeded a long term running testwhere the salt bath of S2-2 is further continuously used for a longperiod of time. Like the tests described above, the long term runningtests were carried out by using the same amount of iron powder and byapplying the same test pieces of iron and steel parts, while thecomposition of the salt bath has been adjusted by supplementing theconsumed component into the salt bath. The processing was conducted fivedays a week, and no processing was made on the weekend. During theweekend, temperature was kept and aeration was maintained.

In the long term running test of two months, the amount of the byproduced cyanide in the salt bath was approximately at 0.5 wt %, and theexternal appearance of the treated metal pieces was black-colored. Theresults of the salt water spray tests indicated that the time untilappearance of rust is more than 200 hours.

However, after three months from the start of the long term runningtest, the center and lower portions of the test pieces became grayishcolor, and the salt water spray tests indicated that the time untilappearance of rust is shortened to 24 hours or less. The result ofchecking the content of the cyanide in the salt bath showed that thecontent is still maintained at around 5 wt %. However, analysis by X-raydiffraction showed that no iron-lithium complex oxide film was detectedon the surface of the test pieces.

The inventors therefore started investigating why the iron-lithiumcomplex oxide film that was formed in the early days did not appearafter the long term running tests by using the salt bath of S2-2, inspite of being constantly maintained the contents of the components ofthe salt bath and the contents of by-produced cyanide. And a part of themolten salt used for the long term running tests was placed as samplesinto a crucible made of titanium having a diameter of 110 mm and a depthof 150 mm. And a method to recover the activity to form the iron-lithiumcomplex oxide film was further investigated.

EXAMPLE 2

The inventors had considered the cause of no formation of theiron-lithium complex oxide film from various points view, whether it isbecause of accumulation of impurities in the salt bath, or whether it isbecause of other reason. As one of the trials, a part of the used moltensalt was taken out and supplemented it with new salt. And aninvestigation was made to find out the suitable ratio to be substitutedby the new salt in order to produce the iron-lithium complex oxideagain.

As a result, it was found that, when only 15 wt % of the molten salt wassubstituted by new salt, then the ability to form the iron-lithiumcomplex oxide revives again. Namely, 15 wt % of the molten salt used forthe long term running tests was replaced with new salt. Then, the carbonsteel of S15C and of the cold rolled steel sheet SPCC were immersed inthe salt bath at 580° C. for 90 min. And it was found that the testpieces thus obtained showed a black-colored appearance and satisfactoryadhesion, which are distinctive of iron-lithium complex oxide. From thisresult, it was considered that the ability to form the iron-lithiumoxide film has been revived. In the salt water spray tests in accordancewith JIS Z2371, it was found that the time until appearance of rust waslonger than 200 hours for these test pieces.

It was supposed that, if the reason of no iron-lithium oxide comes fromthe accumulation of impurities in the salt bath, the amount ofsubstitution of the salt must be at a greater ratio than 15 wt % inorder to revive the ability to form the iron-lithium oxide film.

Then, the inventors speculated that the reason for the revival of theability to form the iron-lithium complex oxide may be related with otherproperties of the newly added salt and not with the old used moltensalt. Based on this speculation, they have expanded the investigation toknow the real factor for the revival. The inventors have paid attentionto the moisture contained in the salt for the supplement use.

Inventors provided a dried salt for the supplement use, which wasprovided by being placed the salt in a oven maintained at 300° C. for 5hours (drying loss in this procedure was 3 wt %) in order to evaporatethe free water in the salt. By using this dried salt, 15 wt % of themolten salt used for the long term running tests was substituted. Thesalt bath was kept at 580° C., and iron pieces of S15C and SPCC wereimmersed therein for 90 min. However, in this case, the iron-lithiumoxide film was not formed, and the iron pieces showed grayish appearancethat is considered to be the nitrided layer. Thus, in this case theability to form the iron-lithium complex oxide was not recovered.

From this result, the inventors thought that the moisture in the saltbath acted to shift the basicity, namely pO²⁻, of the salt bath to thebasic side, thereby enhanced the oxidizing power of the salt bath, andthe ability of the salt bath to form the iron-lithium complex oxide wasrevived.

Incidentally, hydroxide compound such as NaOH, KOH, and LiOH can beexpressed by Na₂O.H₂O, K₂O.H₂O and Li₂O.H₂O, respectively. In order toconfirm the above, the NaOH was added at a rate of 0.3 wt % to the saltbath used for the long term running tests, then S15C and SPCC sampleswere immersed in the salt bath at 580° C. for 90 min. As a result, itwas confirmed that the ability to form the black-colored iron-lithiumoxide film was drastically improved.

Then, a mixture of NaOH, KOH and LiOH prepared by combining each of themat the mol % indicated in Table 1 was added at a rate of 0.3 wt % to thesalt bath used for the long term running tests, and S15 and SPCC sampleswere immersed in the resultant salt bath at 580° C. for 90 min. As aresult, the ability of forming the black-colored oxide film was alsodrastically revived as in the case where NaOH alone was added to thesalt bath.

Test pieces to which the black-colored oxide film was formed were testedby the salt water spray test in accordance with JIS Z2371. As a result,time required until appearance of rust on the surface was found to belonger than 200 hours for all test pieces.

From these results of above, the inventors found out the second reasonof not forming of iron-lithium complex oxide film. As explained before,after three months from the start of the long term running test, thecenter and lower portions of the test pieces became grayish color.However, it was a dry season when three months from the start of thelong term running test in Kanto area where the inventor's laboratoryresides. In the process, air bubbling has been applied to the salt bath.The air used for the air bubbling was natural air without applyinghumidity control thereto. It was understood that, because of lowmoisture content in the air used, the amount of moisture fed to the saltbath was low, which accordingly led to decrease the oxidizing ability ofthe salt bath, thereby causing no formation of the iron-lithium complexoxide film.

Based on this finding, examination was made in order to find out theabsolute moisture content in the air to be preferably used for thebubbling of the salt bath. As a result, it was understood that the useof air with an absolute moisture content of more than (1×10⁻²kg.H₂O)/(1kg dry air), and preferably more than (2×10⁻²kg.H₂O)/(1 kg dry air), iseffective in order to proceed the nitriding and form the iron-lithiumcomplex oxide film onto the surface of the iron parts.

The moisture supply to the salt bath is effective to enhance theoxidizing activity of the salt bath used in the present invention.Therefore, moisture supply by water and by steam may result in the goodeffect. However, it is not preferable because the supply of water orsteam into the molten salts being at a high temperature is dangerous.

As described before, it is advantageous for the formation of theiron-lithium complex oxide film that the amount of the by-productedcyanide in the salt bath is as low as possible. In addition, forminimizing the unfavorable influence against the environment, the amountof the cyanide product in the salt bath should be kept as low aspossible.

As mentioned in the foregoing, the addition of NaOH, KOH, and LiOH intothe salt bath drastically enhances the oxidizing activity of the saltbath (it is presumed that the oxidizing activity of the cyanate in thesalt bath is enhanced due to increase of the basicity in the salt bath).And even when the accumulated amount of the CW in the salt bath exceeded2 wt % level, it is possible to simultaneously form the iron-lithiumcomplex oxide film onto the surface of iron parts simultaneously withthe nitriding.

However, the use of excess amount of alkali hydroxide should be limitedto an appropriate extent since it may accelerate the decomposition of acyanate, the main component for nitriding. (When basicity of the saltbath became high, the decomposition of the cyanate is accelerated.) Theaccumulated amount of CN⁻ in the salt bath is preferably maintained in arange not more than 2 wt %, preferably not more than 1 wt %.

EXAMPLE 3

In the example 2, explanation was made on the cause of loss of theability to form the iron-lithium complex oxide film in the salt bathbeing used for the long term and the means to recover the ability.

The salt bath of the invention is required to be stable for producingiron and steel parts of good and equal quality in order to make theinvention as a commercial process.

In this respect, the inventors have investigated the suitable amount ofsupplemental alkali hydroxide that has a strong influence on the oxidefilm forming ability of the salt bath under the condition of usingmoistened air for the bubbling of the salt bath.

As described in example 2, the amount of the alkali hydroxide added tothe salt bath for recovering the ability to form the iron-lithium oxidefilm was 0.3 wt % when the adding salt was NaOH alone or mixture ofNaOH, KOH and LiOH at the mixing ratio indicated in Table 1.

However, further experiments were continued on the amount of alkalihydroxide to be added. And it was found that an addition of the alkalihydroxide in an amount of 0.005˜0.05 wt % to a total weight of the saltbath for each treatment charge enables the salt bath to make theproducts of good and equal quality.

In order to form the iron-lithium complex oxide film simultaneously withthe nitrided layer, it is required to maintain the content of CN⁻ in thesalt bath at not more than 2 wt %, preferably not more than 1 wt %. Tocomply with this requirement, it is effective to maintain the content ofits parent component, namely CNO⁻, at low.

Inventors have investigated the nitriding performance of the salt bathof the composition of S2-2 in Table 1 in relation with its content ofCNO⁻, and it was confirmed that the nitrided layer with a normalthickness can be obtained when the salt bath contains at least 5 wt % ofCNO⁻. However, when continuous processing are carried on, the content ispreferably not less than 10 wt %.

In the conventional salt bath for nitriding process, the operations arecarried out with the CNO⁻ content at around 35 wt %. In that case,equilibrated CN⁻ content is in a range of 1.about.2 wt % in many cases,though it cannot be fixed to that range since the loss of the salt mayvary depending on the shape and size of the material to be processed.Based on the above, it is required to suppress the upper limit of CNO⁻content at not more than 35 wt %. And in order to maintain the CN⁻content at 1 wt % or less, it is preferable to keep the CNO⁻ content tobe not more than 25 wt % or less.

EXAMPLE 4

In the nitriding process, it is important that the salt bath has acomposition to form more preferable nitrided layer.

In recent years, a nitriding process which arises less thermal stress inthe treated metal is required. Therefore, the salt bath is preferablythe one by which the processing at 450° C. can be realized. On the otherhand, a cyanate has a melting point lower than that of its correspondingcarbonate. And the inventors prepared a mixed salt for a salt bath fornitriding process containing lithium, sodium and potassium and havingsolidifying points of the mixed carbonate of Li, Na and K being to belower than 500° C., and containing CNO⁻ to be at 10 wt %, and thesolidifying points of these samples were measured. The results are shownin Table 3.

TABLE 3 Solidifying temperature of salt containing 10% of cyanate SaltBath for Nitriding Component S1 S2 S3 S4 S5 C1 C2 Li⁺ mol % 25.5 31.020.0 45.0 40.0 30.0 30.0 Na⁺ mol % 45.0 26.5 20.0 25.0 45.0 10.0 55.0 K⁺mol % 30.0 42.5 60.0 30.0 15.0 60.0 5.0 CNO⁻ wt % 10 10 10 10 10 10 10Solidifying 420 378 388 406 427 483 476 point ° C.

The carbon steel of S15C and the cold rolled steel sheet of SPCC wereimmersed in salt bath at 580° C. for 90 min. The compositions of saltbath are shown in Table 3, respectively. Cross sections of the obtainednitrided material were observed with an optical microscope to check thethickness of the compound layers and a thickness of the porous layersformed in the compound layer. The results are shown in Table 4.

TABLE 4 Salt Bath for Nitriding and Obtained Compound Layer Salt Bathfor Nitriding Material S1 S2 S3 S4 S5 C1 C2 SPCC CL 10μ CL 11μ CL 8μ CL10μ CL 11μ CL 4μ CL 15μ PZ 0μ PZ 0μ PZ 0μ PZ 0μ PZ 1μ PZ 0μ PZ 8μ S15CCL 12μ CL 12μ CL 10μ CL 13μ CL 12μ CL 6μ CL 19μ PZ 0μ PZ 0μ PZ 0μ PZ 0μPZ 1μ PZ 0μ PZ 8μ CL: Thickness of the compound layer PZ: Thickness ofthe porous layer in the compound layer

From the results shown in Tables 3 and 4, it was found out that the saltbaths of S1, S2, S3, S4 and S5 are recommendable, because each of thesesalt baths has a solidifying point of lower than 450° C., and thenitriding performance, namely the thickness of the compound layer, ismore than a normal level and having less porous layer. In contrast to S1through S5, salt baths of C1 and C2 are not recommendable sincesolidifying points are higher than 450° C., and the salt bath of C1 isinferior in the thickness of the compound layer, and the property of thenitride layer formed in the salt bath of C2 was inferior since itcontained a thick porous layer.

From the results described above, it was found out preferable to use asalt bath containing alkali components at in a ratio where solidifyingtemperature of the mixed carbonate of Li⁺, Na⁺ and K⁺ falls within arange surrounded by the solidifying temperature line of 500° C. in thephase diagram of carbonates of three elements of Li⁺, Na⁺ and K⁺ asshown in FIG. 3, and wherein the mol ratio of Na⁺ and K⁺ falls within arange from 2:8 to 8:2.

EXAMPLE 5

<Test for Abrasion Resistance>

In the embodiment example, specimens of SPCC being treated in the saltbath of this invention on 8th day of its long term running test ofexample 1 were provided. And the treatment was processed with the saltbath at 580° C. for 90 min.

In the comparative example, specimens of SPCC material being treated bythe conventional nitriding salt bath (TAFTRIDE TF1) were provided. Andthe treatment was processed with the salt bath at 580° C. for 90 min.

Abrasion resistance has been evaluated by measuring the maximum loadwith no scoring defects by using the SRV testing machine and in thecondition as explained below.

Holding time: 60 sec

Step Load: 50N/50 sec

Slide distance: 2 mm

Slide frequency: 50 Hz

Lubricating oil: Base oil for engine oil

TABLE 5 Maximum Load with no Treatment Process Scoring DefectsEmbodiment example 1000, 950, 1000 Comparative example 750, 850, 900

From the results shown in Table 5, it is obvious that the materialsprocessed by the salt bath according to the present invention areprovided with abrasion resistance at least equal to or superior thanthat provided by the conventional nitriding process.

INDUSTRIAL APPLICABILITY

According to the process of the present invention, iron parts havingexcellent corrosion resistance and abrasion resistance can be obtainedby carrying out the single nitriding process without requiring anadditional electrolysis process.

1. A nitriding process of iron and steel parts having an improvedcorrosion resistance, comprising: immersing the iron and steel parts ina salt bath containing a cationic component of Li⁺, Na⁺, and K⁺ andanionic components of CNO⁻ and CO₃ ²⁻, using a bubbling air having anabsolute moisture content of more than (1×10⁻² kg.H₂O)/(1 kg dry air)for mixing the salt bath in order to form an outermost film ofiron-lithium complex on a nitrided layer.
 2. A nitriding process of ironand steel parts according to claim 1, wherein the salt bath contains 3cationic components of Li⁺, Na⁺ and K⁺ in an containing ratio of beingfall the solidifying temperature of the mixed carbonate of Li⁺, Na⁺ andK⁺ within a range surrounded by solidifying temperature contour lines of500° C. in the phase diagram of carbonates of these 3 components, andwherein the mol ratio of Na⁺ and K⁺ falls within a range from 2:8 to 8:2and the content of anionic compounds CNO⁻ is in a range of 5˜35 wt %. 3.A nitriding process of iron and steel parts according to claim 1 whereinthe accumulated content of the by-product cyanide in the salt bath ismaintained at less than 2 wt % in CN⁻.
 4. The nitriding process of ironand steel parts according to claim 1, further comprising adding at leastone hydroxide selected from the group consisting of lithium hydroxide,sodium hydroxide, and potassium hydroxide in an amount of 0.005–0.05 wt.% to a total weight of the salt bath for each treatment charge.